Field of the Invention
The present invention relates to the recovery of heavy metals and rare earth elements from fly ash and coal ash.
Description of the Prior Art
In a single year, over 70 million metric tons of ash is created by coal burning plants. Much of this ash is contaminated with heavy metals, such as lead and arsenic, as well as heavier elements such as the rare earths. It has been calculated that the average rare earth element (REE) of this ash is near 0.1 to 0.05%. If these REE's could be recovered from the coal ash, that would come to nearly 300,000 tons of REE's per year, or three times the amount currently produced by China, the main source of REE's for the world.
Currently, 30-40 million tons of fly ash are stored each year, sometimes in large slurry-filed dams that can rupture and cause wide-spread contamination and devastation. Although some coal ash or fly ash is used in other industries, such as the cement industry, much of the coal ash cannot be used due to the heavy metal contamination, including thorium, which also makes the ash too radioactive to be used in other industries. As such, much of the ash must be stored indefinitely.
The Tennessee Valley Authority Kingston Fossil Plant coal fly ash slurry spill released 1.1 billion US gallons or 5.4 million cubic yards of coal fly ash slurry. River testing of the contaminated area showed significantly elevated levels of toxic metals (including arsenic, copper, barium, cadmium, chromium, lead, mercury, nickel, and thallium).
Unfortunately, there are a limited number of methods that can be used for processing either mineral ores or fly and coal ash, without incurring significant financial burden to industry. As such, the number of coal ash/slurry storage areas is growing across the country. The present invention is the first to use an ionic liquid based approach to remove the heavy metal contamination, not only from fly and coal ash, but also for the removal of rare earth elements from unrefined ores.
In general, previous methods for treating fly ash or mineral ores used different types of acidification in water/slurry mixtures, followed by post processing to remove the heavy metals. This process is known to generate significant amounts of contaminated water, which limits the applicability of these techniques. Additionally, there is significant cost associated with these technologies owing to the need for handling and processing contaminated waste water.
For fly or coal ash, the other most common technique involves the use of either plasma heating the ash to temperatures in excess of 3000° C., which reduces the heavy metals to their metallic state. Once in their metallic state, they can then be recovered from slurry mixtures since they settle out. However, the use of fossil fuels to create the heat required, make this technique too expensive to be applicable for large scale processing. The second technique involves the use of MgCl2 or CaCl2, which is fused with the fly ash at 1000 to 1400° C. Again, this requires large amounts of the chloride salt, which is not regenerated in the process, as well as the energy required to heat the fly ash to high temperatures.